U.S. patent number 3,985,702 [Application Number 05/581,197] was granted by the patent office on 1976-10-12 for dry blending and molding process.
This patent grant is currently assigned to Shell Oil Company. Invention is credited to Glenn Roy Himes.
United States Patent |
3,985,702 |
Himes |
October 12, 1976 |
Dry blending and molding process
Abstract
A process is provided for making a molded article having both a
thermoplastic polymer component and a thermoplastic rubber
component which comprises dry blending small particles of
thermoplastic rubber with particles of thermoplastic polymer having
an average particle size diameter less than about 1.0 mm; and a
liquid plasticizer; and forming the dry blended mixture into a
molded article.
Inventors: |
Himes; Glenn Roy (Torrance,
CA) |
Assignee: |
Shell Oil Company (Houston,
TX)
|
Family
ID: |
24324253 |
Appl.
No.: |
05/581,197 |
Filed: |
May 27, 1975 |
Current U.S.
Class: |
523/167; 524/505;
525/232; 525/98; 525/197; 525/241 |
Current CPC
Class: |
C08J
3/005 (20130101); C08L 57/00 (20130101); C08L
57/00 (20130101); C08L 53/02 (20130101); C08L
91/00 (20130101); C08L 57/00 (20130101); C08L
2666/02 (20130101) |
Current International
Class: |
C08L
57/00 (20060101); C08J 3/00 (20060101); C08K
005/01 () |
Field of
Search: |
;260/33.6AQ,34.2,876B |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Haws et al., Rubber World, 167, 27-30, 32, and 48 (Jan.
1973)..
|
Primary Examiner: Jacobs; Lewis T.
Assistant Examiner: Fletcher; H. H.
Claims
I claim as my invention:
1. The process for making a molded article having a thermoplastic
elastomer component and a thermoplastic polymer component selected
from the group consisting of polystyrene, ethylene vinyl acetate
copolymer, polyethylene, polypropylene and mixtures thereof,
comprising:
a. forming small particles of thermoplastic rubber;
b. forming particles of thermoplastic polymer having an average
particle size diameter less than about 1.0 mm;
c. dry blending the particles from steps a) and b) together with a
liquid plasticizer for at least the rubber or the polymer to
produce a free-flowing particulate mixture;
d. and forming the mixture into a molded article.
2. A process according to claim 1 wherein the thermoplastic
elastomer is a block copolymer of a conjugated diene and a
monalkenyl arene.
3. A process according to claim 1 wherein the thermoplastic polymer
is polystyrene.
4. A process according to claim 1 wherein the plasticizer is a
hydrocarbon oil.
5. A process according to claim 1 wherein the thermoplastic polymer
particles formed in step (b) have an average particle size between
about 0.15 and 0.75 mm.
6. A process according to claim 1 for making molded footwear
component comprising:
a. forming small particles of a thermoplastic elastomeric block
copolymer of styrene and butadiene;
b. forming particles of polystyrene having an average particle size
diameter between about 0.15 and 0.75 mm;
c. dry blending the block copolymer and polystyrene with a
plasticizing proportion of a mineral oil; and
d. molding the mixture so formed.
7. The process for making a molded article having a blended
thermoplastic elastomer component and a thermoplastic polymer
component which comprises
a. dry blending particles of a thermoplastic polymer, selected from
the group consisting of polystyrene, ethylene vinyl acetate
copolymer, polyethylene, polypropylene and mixtures thereof, the
particles being less than about 1 mm average diameter, a
plasticizer and small particles of a thermoplastic elastomer to
form a free flowing particulate mixture; and
b. forming the mixture into a molded article.
8. A process according to claim 7 wherein the thermoplastic polymer
is polystyrene, the average particle diameter of which is between
about 0.15 and 0.75 mm.
9. In a process for molding an article from a mixture of a
thermoplastic polymer and a thermoplastic elastomer, wherein a
plasticizer and small particles of both said rubber and polymer are
dry blended to form a free flowing particulate mixture and forming
the mixture into a molded article, the improvement comprising using
particles of the thermoplastic polymer having an average particle
size diameter less than about 1 mm.
Description
BACKGROUND OF THE INVENTION
This invention relates to the manufacture of molded articles. The
use of elastomeric materials such as synthetic rubber in the
manufacture of molded articles such as shoe components has long
been known. Such materials have high coefficients of friction and
ground-gripping properties and are very flexible at low or ambient
temperatures. With most types of rubber, however, it is necessary
to compound them with vulcanizing agents and subject the compounds
to mastication followed by molding operations in the presence of
sufficient heat and for a time sufficient to effect vulcanization.
Of course, for some end uses, other polymers such as
polyvinylchoride may be employed. The latter type of polymer,
however, exhibits certain undesirable characteristics for end uses
such as footwear due for example, to high wet slip and cold
stiffness.
In recent years a number of block polymers have been developed
which are elastomeric and which exhibit the stress-strain
properties of vulcanized rubber without having been chemically
vulcanized. This special class of polymers is referred to as
thermoplastic elastomers, since they not only exhibit typical
elastomeric properties but also may be processed in equipment
normally employed for the rocessing of ordinary thermoplastic
polymers such as polystyrene processing polyolefins.
The most highly developed types of block copolymers include linear
or branched species having thermoplastic polymer blocks
interspersed with elastomeric polymer blocks. Typical examples of
these include block copolymers of monoalkenyl arenes with
conjugated dienes as well as certain hydrogenated derivatives
thereof. Other suitable thermoplastic elastomers comprise olefinic
block polymers having blocks of ethylene or propylene combined with
olefin copolymer blocks. Typical commercial materials include those
sold under the trade name "KRATON" by Shell Chemical Company and
"KRATON G".sup.tm, sold by the same company. The latter block
copolymers comprise polymer blocks of monoalkenyl arenes such as
styrene combined with elastomeric polymer blocks of an olefin
copolymer or a hydrogenated polymer block of a conjugated
diene.
Suitable methods for the preparation of block copolymer
thermoplastic elastomers may be found in the following patents:
U.s. pat. No. 3,265,765
U.s. pat. No. 3,333,024
U.s. pat. No. Re 27,145
U.s. pat. No. 3,594,452
U.s. pat. No. 3,726,944
U.s. pat. No. 3,244,664
Because of their desirable combination of physical properties,
these thermoplastic elastomers have found ready acceptance in
mumerous molding processes and particularly in the shoe industry as
well as pharmaceutical, automotive and sporting equipment. In the
past, the thermoplastic elastomers have generally been compounded
with other components such as fillers, extenders, plasticizers and
the like in a kneading-type of mixer such as a Banbury Mixer. Such
mixers entail high power requirements and the compounded mixtures
must thereafter be further treated to form nibs or pellets, which
are then utilized in well-known molding machines such as injection
molders and the like.
Another type of compounding process which is beginning to receive
attention (see U.S. Pat. No. 3,877,101) is referred to as
dryblending. By this is generally meant a low shear mixing process
which blends the compounding ingredients together without the
formation of a coherent mass and which results in a relatively
free-flowing mixture of ingredients in particulate form--which
forms a suitable feed for commercially available molding
equipment.
Certain difficulties have been encountered in adapting dry blending
to thermoplastic elastomer compounds wherein the compounds include
additionally not only a plasticizer but also thermoplastic polymers
such as polystyrene, polyethylene or polypropylene, and other known
thermoplastic polymers. In many instances it has been found that
dry blending has not accomplished the desired degree of dispersion
of the thermoplastic polymer in the molded article. This may result
in certain unsatisfactory physical properties in the end
product.
In the present specification and claims, the term "thermoplastic
polymer" is distinguished from the term "thermoplastic elastomer"
or "thermoplastic rubber" in that the thermoplastic polymers are
defined as non-elastomeric polymers, which have the generally
well-known characteristics of ordinary thermoplastics such as those
referred to hereinbefore, namely, polystyrene, polyethylene or
polypropylene.
It is an object of this invention to provide an improved process
for manufacturing molded articles utilizing combinations of
thermoplastic rubbers, plasticizers and thermoplastic polymers. It
is a further object of this invention to provide a process for
utilizing a dry blending process for compounding these ingredients
in which the subsequently molded article exhibits improved
dispersion of the compound ingredients. Other objects will become
apparent during the following detailed description of the
invention.
DESCRIPTION OF THE INVENTION
Now, in accordance with the present invention, a process for making
a molded article from a thermoplastic rubber, a thermoplastic
polymer, and a plasticizer comprises the following essential
steps:
a. dry blending small particles of thermoplastic rubber with
particles of thermoplastic polymer having an average particle size
diameter less than about 1.0 mm and a liquid plasticizer; and
b. forming the mixture into a molded article.
In accordance with the present invention it was found that the
average particle size diameter of the thermoplastic polymer was
more critical in the dry blending process than was the average
particle size of the thermoplastic elastomer. As will be seen in
the working examples which follow, optimum results were obtained
when the thermoplastic polymer e.g., polystyrene, enthylene vinyl
acetate copolymers, polyethylene or polypropylene, and mixtures
thereof, was formed in particles having average diameters between
0.15 and 0.75 mm. This is evidenced by the inprovement in physical
properties of injection molded shoes made from a number of dry
blended compounds wherein a variable between the several samples
was the average particle size of the polystyrene. The degree of
ultimate dispersal of blend components is equally important in
molded articles intended for other purposes.
In forming small particles of either the elastomer or polymer, it
is important to effect the particle-size reductions in such a
manner as to minimize the possibility of thermal and mechanical
degradation. Preferably this is done by granulating the two types
of materials in high shear, low impact, rotary cutting granulators
such as those sold by Wedco, Inc., Cumberland Engineering Co.,
Entoleter, Inc., Amacoil Machinery, Inc., and Metalmecconica Plast.
S.A. of Italy, and others which are known in the art. Preferably,
the thermoplastic elastomer is reduced to a particle below 3/16 of
an inch, preferably to pass a standard 10-30 mesh screen. These
particles, of course, may be inherently formed in the
polymerization process in the correct particle size. Another means
of forming the small particles comprises addition of a non-solvent
for a rubber or polymer solution in such a way as to form a
powdered precipitate. For economic reasons it is preferred to
utilize the largest particle size which will give the desired
degree of dispersion in the eventually molded article. The optimum
particle size may vary with the specific molding apparatus and
molding conditions used. However, as stated hereinabove, the
particle size of the thermoplastic polymer has been found to be
unexpectedly critical in achieving the desired degree of
dispersion, whereas substantially more leeway is possible with the
particle sizes of the thermoplastic elastomers. Thermoplastic
elastomers are often porous bodies due to their method of recovery
from their polymerization mixtures in the form of a crumb. This
porous structure is retained in the ultimate particles formed in a
granulating operation.
The essential step of the process of the present invention is the
dry blending (also known as dry mixing) of the suitable
thermoplastic elastomeric particles, thermoplastic polymer
particles, and plasticizer, together with other special compounding
ingredients such as fillers, resins, colorants, antioxidants and
the like. Dry blending has been employed previously for compounding
polyvinyl chloride compounds, and recently has been developed to a
certain extent in compounding thermoplastic elastomers as shown by
U.S. Pat. No. 3,877,101.
The equipment for dry blending is essentially the same as that
employed in the dry blending of polyvinyl chloride. While the
equipment preferably combines mixtures of the high intensity types
such as the German Henschel or Pappenmeier types, better known as
the Prodex or Welex machines, respectively, in the United States.
Other suitable apparatus is also known in the art. Less intense
types of dry blending equipment have been found unexpectedly
suitable, such as drum tumblers, ribbon blenders or rotary mixers
such as cement mixers.
Generally, the ingredients may be mixed with plasticizers such as
hydrocarbon (i.e. petroleum) extending oils, by several stages, or
in a single stage. Due to their affinity, it is preferred that at
least a portion of the plasticizing oil be added to the
thermoplastic elastomer prior to incorporation of the balance of
the compounding ingredients. In fact, it is possible to blend at
least a portion of the oil or other plasticizer with the
thermoplastic elastomer before forming the small particles of the
latter. Alternatively, the thermoplastic elastomer may be placed in
the mixer together with the plasticizer and mixing commenced. In a
relatively short space of time, it is found that the plasticizer is
absorbed on the particles of the compound components and the dry
blending process eventually results in a relatively free flowing
particulate mixture which is suitable for use in subsequent molding
opeartions.
A wide variety of compounding agents or additives suitable for
thermoplastic elastomer compounds are well known in the art.
Fillers and reinforcing agents such as clays, carbon blacks,
silicas, whitings and others often improve resistance to abrasion
and crack growth and increase hardness. Oils generally act as
plasticizers, with naphthenic and paraffinic types being preferred
to aromatic oils. Other known plasticizers can also be used. Fire
retardants may be utilized. Other resins may advantageously be
combined with the thermoplastic elastomer to adjust the properties
of the final product. Polystyrene, for example, is useful to adjust
properties and is quite compatible with butadiene-styrene block
polymers. The addition of polystyrene generally increases hardness,
tear strength, abrasion resistance and flex life. Other resins
useful alone in combination with polystyrene are known in the art,
and include polyindine, coumarone-indene, pentaerythritol esters of
hydrogenated resin, etc. Other suitable ingredients, such as
antioxidents, stabilizers, etc., are also known in the art.
The granulating and dry blending of thermoplastic elastomers and
similar materials is shown in the patent and open literature, such
as in the following patents:
______________________________________ Filing Date U.S. 3,589,036
1-06-65 U.S. 3,769,417 5-12-71 U.S. 3,793,283 3-16-72 U.S.
3,558,576 2-06-68 U.S. 3,567,670 12-29-67
______________________________________
A number of Technical Bulletins published by Shell International
Chemical Company Ltd., London, also show a number of aspects of
granulating and dry blending thermoplastic elastomers,
thermoplastic polymers and plasticizers. It is evident from these
that there is no unexpected difficulty in granulating block
copolymers or their compounds and that dry blending is recommended
to avoid Banbury or extrusion blending. Some of these Bulletins are
listed herewith:
______________________________________ Publication Date RBX/72/19
Nov., 1972 RBX/73/3 Feb., 1973 RBX/72/24 Sept., 1972 RBX/71/10
Mar., 1971 RBX/72/4 Jan., 1972 RBS/72/5 Mar., 1972 RBX/71/27 Sept.,
1971 RBX/69/44 1969 ______________________________________
While elevated temperatures tend to aid the absorption of the oils
into the elastomer and thus speed the dry blending step, care
should be taken that the temperature not be allowed to rise too
high. Normally the dry blend is in condition for use long before
temperatures would climb to degrading levels, but in any event the
temperature should not be allowed to exceed 170.degree. F, more
preferably should be maintained below to 150.degree. F.
The final essential stage in the present process comprises forming
the dry blended mixture into a molded article. The formation of
footwear is especially contemplated, and injection molding of
footwear is particularly recommended. However, the dry blended
compounds may be treated by other molding opeartions such as
extrusion or calendering. The formation of slab soling, for
instance, results in an extruded or calendered sheet from which
footwear soles or heels may be stamped. Reference is made to U.S.
Pat. No. 3,589,036 for sutiable variations in compounding and
footwear formation. Suitable commercial footwear injection molding
machines include those manufactured by Desma, Bata, and Gesta. It
was found in accordance with the present invention, that the
polystyrene dispersion and the crack resistance varied with the
average particle size of the polystyrene in the footwear
compound.
The following example presents details of a comparative study of
polystyrene particle sizes and its effect upon molded articles. The
following formulation was prepared in the dry blending process for
these comparative tests:
TABLE I ______________________________________ Parts by weight
Oiled block polymer .sup.1) 155 Mineral Oil 53 Crystal polystyrene
60 Calcium carbonate 39 Clay 39 Titanium dioxide 7 Antioxidant 1
______________________________________ .sup.1)
Polystyrene-polybutadiene-polystyrene, block mol wts
22,000-48,000-22,000, the oiled polymer contained 55 parts by wt.
of a paraffinic/naphthenic rubber-extending mineral oil per 100
parts by weigh of the block copolymer.
The oiled block copolymer was granulated to an average particle
diameter of 1.7 mm in a Wedco Granulator to pass a 12-mesh U.S.
Standard Sieve. A series of polystyrene samples was granulated in
the same type of apparatus from the original nib size (2.4 mm
diameter) to the particle sizes listed in the Table II given below.
The oiled block copolymer and the supplementary
paraffinic/naphthenic-extending oil were then mixed in a cement
mixer at about 115.degree. F, after which the remaining dry
ingredients were added and mixing continued for about 20 minutes to
produce the relatively free flowing particulate dry blended
compounds. Each of these compounds was then utilized in a Bata
Monopak injection molding shoe machine wherein canvastop shoes were
made. The following Table II illustrates the effect of polystyrene
particle size upon the quality of the shoes so obtained.
According to the data obtained, polystyrene dispersion was poor
when the particle size was greater than about 1 mm. Moreover, this
is reflected in the relatively poor flex life of the shoes made
from the coarse samples of polystyrene. In addition, sprue breakage
occured with shoes containing the coarser polystyrene.
TABLE II
__________________________________________________________________________
Dry Blends Predominant Particle Size of 2.4 1.3 0.6 0.4 0.3 0.17
Polystyrene Used (minimum (nibs) dimension, mm. Thin Sheet
Dispersion of Very Poor Fair Accept. Accept. Accept.
Polystyrene.sup.(2) Poor Hardness, Rex 58 63 56 59 60 60 Stiffness,
manually rated, 5 5 5 3 3 3 1 to 6 scale (6=stiffest) Ross Flex
Crack Resistance at 75.degree.F (0.257 inch thickness) Kilocycles
to rating.sup.(3) of 9 40 54 90 90 84 148 6 72 99 170 171 141 217 2
149 218 200 245 195 366
__________________________________________________________________________
.sup.(1) Molded on the Bata Monopak machine. .sup.(2) Acceptable:
no visible undispersed polystyrene; Fair: few small fragments or
streaks. Poor: many small particles; Very Poor; many large
particles of original nib size. .sup.(3) Based on 10 to 0 scale in
which 10 denotes no cracking and 0 denotes complete rupture. ASTM
Test D1052-55. Modified by omitting aging (paragraph 3.2).
* * * * *